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spritesmith-engine-spec

Specification for spritesmith engines

In addition to this repo, we offer an integration test suite via spritesmith-engine-test.

Version

This documentation is for version:

2.0.0

Documentation

Terminology

In spritesmith, the following terms and definitions will be used:

  • engine - A library that handles interactions between images
  • image - Object containing metadata about an image file or buffer
  • canvas - Class that handles placing images onto a visual layer and generating an output image

Process

When spritesmith is generating a spritesheet, we will perform the following steps:

  1. Process images into metadata (e.g. get size of each image)
  2. Calculate layout based on user preferences (outside of engine)
  3. Create canvas to place images on
  4. Place each image on canvas
  5. Export canvas as an image

Exports

A spritesmith engine returns an Engine constructor function with as its module.exports

new Engine(options)

Constructor for a new engine

  • options Object - Container for flags to set for the engine
    • For example, gmsmith has imagemagick: true to force usage of ImageMagick over GraphicsMagick

Engine.specVersion

String representing current specification version the engine is supporting (e.g. '1.0.0')

engine.createCanvas(width, height)

Function to create a new canvas based on the engine. This should have the function signature (width, height, cb)

  • width Number - Width in pixels for the canvas
  • height Number - Height in pixels for the canvas

Returns:

  • canvas Canvas - Instance of a canvas class for the engine

Note: This method is intentionally synchronous. Please run all asynchronous actions during canvas.export. We suggest saving any critical metadata to canvas and reusing it during canvas.export.

engine.createImages(images, cb)

Function to create images which will later be laid out via a canvas. This should have the function signature (images, cb)

  • images String[]|Object[] - Array of filepaths to images or vinyl representations themselves
    • If an image is a String, then its the filepath to the image
    • If an image is an Object, then its a Vinyl object describing the image (e.g. filepath, contents)
      • image may have a Buffer, Stream, or null contents. Please handle each of these cases appropriately (e.g. warn users when contents isn't used)
  • cb Function - Error-first callback function to return image metadata via
    • cb will have the function signature (err, images)
    • If there is an error, run cb(err). Otherwise, callback with an array of image metadata (i.e. cb(null, images))
    • This should be called asynchronously (e.g. if creation is synchronous, use process.nextTick)
    • image Object - Metadata container about corresponding input image at same index
      • height Number - Height in pixels of corresponding input image at same index
      • width Number - Width in pixels of corresponding input image at same index
      • Any other metadata can be stored here and will be passed to canvas.addImage (e.g. filepath)

new Canvas(width, height, engine)

Placeholder documentation to suggest how to write a canvas. It's possible to avoid using a constructor but it will lead to more trouble than not.

The following methods are required as part of the returned Canvas object

canvas.addImage(image, x, y)

Function to add an image onto our canvas. This should have the function signature (image, x, y)

  • image Object - Image object created via engine.createImages
    • This will be the same object so any additional metadata will be accessible (e.g. filepath)
  • x Number - Horizontal coordinate to position left edge of image
  • y Number - Vertical coordinate to position top edge of image

Note: This method is intentionally synchronous. Please run all asynchronous actions during canvas.export. We suggest saving any critical metadata to canvas and reusing it during canvas.export.

canvas.export(options)

Function to export canvas as an image. This should have the function signature (options, cb)

  • options Object - Modifiers to indicate how to export (e.g. format, quality)
    • format String - Image format to export canvas as
      • Most engines should default to png
      • Commonly used formats are: png, jpeg, and gif
    • Any other options can be defined custom to your engine (e.g. quality)

Returns:

  • resultStream Object - Readable stream with exported image being written out via Buffer on data events

Discovery

In order to make our engines easily discoverable, please provide a spritesmith-engine keyword. Here's a list of existing spritesmith engines:

https://www.npmjs.com/browse/keyword/spritesmith-engine

Contributing

In lieu of a formal styleguide, take care to maintain the existing coding style.

Donating

Support this project and others by twolfson via donations.

http://twolfson.com/support-me

Unlicense

As of Oct 28 2015, Todd Wolfson has released this repository and its contents to the public domain.

It has been released under the UNLICENSE.